The present invention is generally directed to an attitude control system for a spacecraft and, more specifically, to controlling a control moment gyro array with a reaction wheel assembly control unit.
Presently, there are numerous three axis stabilized spacecraft (e.g., satellites) that use reaction wheel assemblies (RWAs) for attitude control. Due to the complexity of spacecraft attitude control, the monetary investment in software for a typical RWA control unit is generally quite large. Typically, signals provided by an inertial sensor as well as reference signals provided by other sensors (e.g., earth, sun and star sensors), incorporated within the spacecraft, are utilized by the RWA control unit to determine the effect on the spacecraft when a speed of a rotor, i.e., a wheel, of an RWA has been changed. In general, each sensor and wheel has included an interface, which allows the sensors and wheels to provide information to and/or receive information from the RWA control unit in a designated format (e.g., 1553 protocol). In this manner, the RWA control unit can appropriately control the telemetry of the spacecraft.
To achieve attitude control of the spacecraft, a typical RWA control unit provides torque signals to each RWA associated actuator (e.g., motor/generator), which cause an associated rotor, i.e., wheel, to turn at a desired speed, which wheel speed is, in turn, provided to the RWA control unit. In general, a wheel speed (xcfx89) is related to a torque command (xcfx84) by the following equation:   ω  =      τ          J      ·      S      
where J is the moment of inertia of a given RWA wheel and S is the Laplace Operator.
In general, a torque command is received by an RWA interface, which provides an appropriate signal to the motor and also provides feedback to the RWA control unit, in the form of an associated wheel speed. Each RWA may have it own RWA interface or a centralized interface may be utilized for all of the RWAs. The speeds received by the RWA control unit are utilized, in part, to determine an appropriate external torque for the satellite, so as to desaturate stored angular momentum of the RWA array. Various desaturation techniques may be implemented, such as magnetic torquer bars that produce a torque by reacting against the earth""s magnetic field or hot or cold gas thrusters may be used in pairs to produce a desired torque. While many spacecraft would benefit from the increased agility provided by control moment gyros (CMGs), many spacecraft manufacturers have not gravitated to CMG arrays due to the large monetary investment incurred in writing and refining software to control RWA arrays.
Thus, it would be desirable to provide an attitude control system for a spacecraft that is capable of utilizing a reaction wheel assembly (RWA) control unit to control a control moment gyro (CMG) array.
The present invention is directed to an attitude control system for a spacecraft that includes an attitude control assembly (ACA) interface, a control moment gyro (CMG) array and a reaction wheel assembly (RWA) control unit. The CMG array includes a plurality of CMGs, which are coupled to and receive CMG gimbal rates from the ACA interface and provide CMG gimbal angles to the ACA interface. The RWA control unit is coupled to the ACA interface and provides RWA torque commands to the ACA interface. The ACA interface converts the torque commands to CMG gimbal rates and receives CMG gimbal angles from the CMGs and converts the CMG gimbals angles into RWA speeds, which are provided to the RWA control unit.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.